@idefix37 With the sider all the way down, windy shows isobars (isolines of pressure) at sea level.
With the slider moved up to 500mb, I expect windy to show a chart of isobars (isolines of pressure) representing conditions in the mid- troposphere. This level is at an altitude of approximately 5,500 meters (18.000 ft), relative to a standard atmospheric pressure of 1013.25mb at sea level.
Windy does display this chart, if geopotential height is selected. The isobar labels then represent pressure as equivalent altitude in feet.
Not much use to a sailor. I want to know how to display isobar (isolines of pressure) values in mb instead of feet.
Here is an example standard 500mb (500hPa) chart from ECMWF.